A light-emitting element having a layer containing a luminescent substance between a pair of electrodes is used as a pixel, a light source, or the like, and is provided in a light-emitting device such as a display device or a lighting system. When a current flows between a pair of electrodes in a light-emitting element, fluorescence or phosphorescence is emitted from an excited luminescent substance.
In comparison with fluorescence, theoretically, internal quantum efficiency of phosphorescence is three times as high as that of fluorescence in the case of current excitation. Therefore, it is considered that higher emission efficiency is obtained by using a luminescent substance emitting phosphorescence than using a luminescent substance emitting fluorescence; thus, a substance emitting phosphorescence has been developed so far.
For example, a metal complex where a central metal is iridium is mentioned in Non-Patent Document 1. According to the Document, this metal complex can be used as a material for a light-emitting element.
In the case of a light-emitting element that emits light by current excitation, it is insufficient that only a current flows, and it is necessary to recombine electrons and holes effectively. By increasing recombination efficiency of electrons and holes, excitation energy is efficiently generated, and thus, emission efficiency is improved.
So far, it has been widely known and developed that the substance having high internal quantum efficiency is used to increase emission efficiency; however, a substance capable of increasing recombination efficiency has not yet been sufficiently developed.
[Non-Patent Document 1]
“New Iridium Complexes as Highly Efficient Orange-Red Emitters in Organic Light-Emitting Diodes”, Jiun-Pey Duan et al., Advanced Materials 2003 15 No. 3, February 5, pp 224-228